Detecting bypassed hydrocarbons in subsurface formations

Abstract

A tool for use in analyzing a subsurface formation for the presence of hydrocarbon includes a neutron generator capable of generating a burst of neutrons at a high energy level, some of which will collide inelastically with atomic nuclei in the subsurface formation to produce inelastic gamma rays and then will be captured by atomic nuclei to produce capture gamma rays. The tool also includes at least one radiation detector to detect the inelastic and capture gamma rays, and counting circuitry configured to produce a count of detected inelastic gamma rays and a count of detected capture gamma rays. Processing circuitry in the tool generates a numerical output by dividing the count of inelastic gamma rays by the count of capture gamma rays and then provides the numerical output to an output device for use in analyzing the subsurface formation for the presence of hydrocarbon.

What is claimed is: 
     
       1. A method for use in analyzing a subsurface formation for the presence of hydrocarbons,the method compromising: emitting from a neutron source into the formation a short burst of neutrons at a high energy level, some of which collide inelastically with atomic nuclei in the formation and then move through the information at lower energy level;   detecting gamma rays caused by the inelastic collisions during the short burst;   emitting from the neutron source into the information a long burst of neutrons at the high energy level, some of which collide inelastically with atomic nuclei in the formation and then move through the formation at a lower energy level;   detecting gamma rays caused by the inelastic collisions during the long burst;   detecting gamma rays generating when the neutrons are captured by the atomic nuclei in the subsurface formation;   determining a value represent a microscopic thermal nuetron captured-cross section (SIGMA);   determining a value representing an average baseline sigma (ASV);   generating a numerical output as a function of SIGMA, ASV, the number of inelastic gamma rays detected during the short burst, and the number of ineelastic gamma rays detected during the long burst; and   using the numerical output to determine the presence of hydrocarbons in the formation.   
     
     
       2. The method of claim 1, wherein the gamma rays are detected by at least two radiation detectors. 
     
     
       3. The method of claim 2, wherein the step of generating the numerical output further comprises the step of determining a ratio (IRAT) of inelastic gamma rays detected by the detector located farther from the neutron source and the detector located closer to the neutron source. 
     
     
       4. The method of claim 3 wherein the step of generating the numerical output further comprises the step of determining an average baseline value (ABR) of the ratio (IRAT). 
     
     
       5. The method of claim 4 wherein the numerical output is generated using IRAT, ABR, ASV, and SIGMA. 
     
     
       6. The method of claim 5 wherein the step of generating the numerical output further comprises the step of performing the following operation: (IRAT-ABR)*(ASV/SIGMA). 
     
     
       7. The method of claim 5, wherein ABR represents the average baseline value in a non-hydrocarbon bearing zone in the formation and ASV represents the average baseline sigma in a non-gas bearing shaly sand. 
     
     
       8. The method of claim 2, wherein the step of generating the numerical output further comprises the step of determining a total number of inelastic gamma rays (INFD) detected by the detector located farther from the neutron source. 
     
     
       9. The method of claim 8 wherein the step of generating the numerical output further comprises the step of determining an average baseline value (ABV) of inelastic gamma rays detected by the detector located farther from the neutron source. 
     
     
       10. The method of claim 9 wherein the numerical output is generated using INFD, ABV, ASV, and SIGMA. 
     
     
       11. The method of claim 10, wherein the step of generating the numerical output further comprises the step of performing the following operation: (INFD-ABV)*(ASV/SIGMA). 
     
     
       12. The method of claim 10 wherein ABV represents the average baseline value in a non-hydrocarbon bearing zone in the formation and ASV represents the average baseline sigma in a non-gas bearing shaly sand. 
     
     
       13. The method of claim 1 wherein: the step of detecting gamma rays generated when the nuetrons are captured by atomic nuclei in the subsurface formation takes place during the short burst.   
     
     
       14. The method of claim 1 wherein: the step of detecting gamma rays generated when the nuetrons are capture by atomic nuclei in the subsurface formation takes place during the long burst.   
     
     
       15. A program, stored in a storage medium, for use in analyzing a subsurface formation for the presence of hydrocarbons, the program comprising executable instructions that enable a computer to: acquire a count of inelastic gamma rays detected as a result of irradiating the formataion with burst of high energy from a neutron source;   acquire a count of gamma rays generated when the neutrons are captured by atomic nuclei in the subsurface formation;   acquire a value representing the macroscopic thermal neutron capture-cross section (SIGMA);   acquire a value representing an average baseline sigma (ASV); and   generate a numerical output as a function of SIGMA, ASV, and the inelastic count to determine the presence of hydrocarbons in the formation.   
     
     
       16. The program of claim 15, wherein the count includes the inelastic gamma rays detected by a detector located farther from the neutron source (INFD). 
     
     
       17. The program of claim 16, wherein the count rate includes the inelastic gamma rays detected by a detector located closer to the neutron source. 
     
     
       18. The program of claim 17, further comprising an instruction that enables the computer to generate a ratio (IRAT) of inelastic gamma rays detected by the detector located farther from the neutron source and the detector located closer to the source. 
     
     
       19. The program of claim 18, further comprising an instruction that enables the computer to retrieve an average baseline value (ABR) of the ratio IRAT. 
     
     
       20. The program of claim 19, wherein the numerical output is further a function of ABR and IRAT. 
     
     
       21. The program of claim 16, further comprising an instruction that enables the computer to retrieve an average baseline value (ABV) of inelastic gamma rays detected by the detector located farther from the source. 
     
     
       22. The program of claim 21, wherein the numerical output is further a function of ABV and INFD. 
     
     
       23. A tool for use in analyzing a subsurface formation for the presence of hydrocarbon, comprising: a source capable of emitting into the subsurface formation a burst of neutrons at a high energy level, some of which collide inelasticallly with atomic nuclei in the formation and then move through the formation at a lower energy level;   a detector capable of detecting gamma rays caused by the inelastic collisions during the burst and gamma rays generated when the neutrons are captured by atomic nuclei in the subsurface formation; and   a circuit configured to: generate a value representing the macroscopic thermal neutron capture-cross section (SIGMA);   generate a value representing and average baseline sigma (ASV); and,   generate a numerical output as a function of SIGMA, ASV, and a number of gamma rays detected during the burst.     
     
     
       24. The tool of claim 23 wherein the tool comprises at least two detectors. 
     
     
       25. The tool of claim 24 wherein the circuit provides a value representing a ratio (IRAT) of inelastic gamma rays detected by the detector located farther from the neutron source and the detector located closer to the source. 
     
     
       26. The tool of claim 25 wherein the circuit provides a value representing the average baseline (ABR) of the ratio (IRAT). 
     
     
       27. The tool of claim 26 wherein the circuit generates the numerical output using IRAT, ABR, ASV, and SIGMA. 
     
     
       28. The tool of claim 24 wherein the circuit provides a value representing a total number of inelastic gamma rays (INFD) detected by the detector located farther from the source. 
     
     
       29. The tool of claim 28 wherein the circuit provides a value representing an average baseline (ABV) of inelastic gamma rays detected by the detector located farther from the source. 
     
     
       30. The tool of claim 29 wherein the circuit generates the numerical output using INFD, ABV, ASV, and SIGMA.